Cell Communication: Signaling Pathways & Types

Cell Communication Overview

• Cell communication involves stimulus, signal reception, and response in sequence.
• Signaling molecules, or ligands, transmit messages between cells by fitting into receptor proteins.
• Quorum sensing in prokaryotes depends on cell density, triggering actions like bioluminescence or DNA uptake.
• Adrenaline secretion facilitates cell communication for the fight-or-flight response.
• Endocrine signaling involves long distances and uses the circulatory system to transport signaling molecules.
• Paracrine signaling occurs locally between neighboring cells through diffusion.
• Autocrine signaling happens when a cell signals to itself.
• Direct contact between cells is required for some types of cell communication.
• Gap junctions connect animal cells, allowing direct communication, while plasmodesmata connect plant cells, enabling the sharing of molecules and organelles。
• Cell communication coordinates functions to maintain homeostasis, which includes dynamic processes for steady internal conditions.
• Sensors detect changes from the set point in a homeostatic system.
• Effectors help restore the set point.
• Homeostasis is usually kept by negative feedback.

Signal Transduction

• Cells signal from the outside and receptors on the cell surface receive signals.
• The receptor protein changes shape upon receiving a signal starting a cascade.
• Ligands are signaling molecules that bind to receptors in a lock-and-key manner.

G Protein-Coupled Receptors

• G protein-coupled receptors use a G protein to transmit messages.
• A protein receives a signal, changes shape, and binds to the G protein.
• The energized G protein delivers the message to the target protein, activating the cell's response.
• cAMP spreads information as a secondary messenger inside the cell.
• cAMP spreads through the cell and binds to kinase.
• Kinases add phosphate groups to molecules like proteins, lipids, and nucleic acids through phosphorylation.
• Kinases remove a phosphate group from ATP, a high-energy molecule, creating cAMP.
• Each cAMP molecule binds to a kinase, which in turn activates proteins.

Receptor Protein Kinase

• Receptor protein kinases span the membrane and activate when two halves of the protein come together.
• Phosphate groups are added to activate proteins.

Ligand-Gated Ion Channels

• In ligand-gated ion channels, ligand attaches to a protein, enabling ions to flow through.
• The ligand then detaches, closing the protein.

Hydrophobic Signals

• Hydrophobic signals directly pass through the cell membrane to deliver messages inside the cell and activate proteins.

Cell Cycle Stages

• Prophase: The nucleus breaks down, chromosomes condense, and spindles start to form.
• Prometaphase: The cell prepares to separate chromosomes.
• Metaphase: Chromosomes align in the center of the cell.
• Anaphase: Sister chromatids separate and move to opposite poles.
• Telophase: New nuclear membranes form, and chromosomes decondense.
• Cytokinesis follows mitosis.

Regulation of the Cell Cycle

• Control of cell division involves signaling molecules that regulate cell cycle phases.
• Cyclins are signaling proteins discovered in mitosis studies.
• Cyclin concentration increases during interphase, peaks in mitosis, and decreases after cell division, beginning to build up again in the next interphase.
• G1 phase gathers material for DNA replication, S phase replicates DNA, G2 prepares for mitosis, and G0 is a pause.
• Cyclins and cyclin-dependent kinases (CDKs) control the cell cycle.
• Kinases, activated by cyclins, phosphorylate and activate proteins.
• Checkpoints occur before each phase of the cell cycle; if issues cannot be fixed, the cell undergoes apoptosis.

Cell Cycle Checkpoints

• Checkpoint 1: Occurs at the end of G1 to check for DNA damage.
• Checkpoint 2: Occurs at the end of G2 to ensure correct DNA copying.
• Checkpoint 3: Occurs during mitosis (after metaphase, before anaphase) to confirm correct spindle alignment.
• A damaged cell that replicates can cause proliferation, forming a tumor.
• Necrosis involves cell membrane breakdown, releasing contents into surrounding areas and signaling cell death, which can affect other cells.

P53 Protein

• P53 is a protein in the nucleus that activates when DNA damage is present.
• P53 creates proteins that halt cell cycle progression.
• When P53 is not working correctly, it cannot signal the cell to stop dividing.

Cell Communication Distances

• Endocrine signaling is long-distance, with hormones transported through blood vessels.
• Paracrine signaling is short-distance, with signals released into surrounding fluid and diffused to nearby cells.
• It operates in the central nervous system.
• Autocrine signaling involves a cell releasing and receiving its own signals, determining cell fate.